1. Field of the Invention
The present invention relates generally to the production of steam from silica-rich geothermal brine and especially to processes for using silicious sludge obtained from silica-rich geothermal brine as a by-product of such steam production.
2. Discussion of the Prior Art
Large subterranean aquifers of naturally produced (geothermal) steam or hot aqueous liquids, specifically water or brine, are found throughout the world. These aquifers, which often have vast amounts of energy potential, are most commonly found where the earth's near-surface thermal gradient is abnormally high, as evidenced by unusually great volcanic, fumarole or geyser activity, Thus, as an example, geothermal aquifers are not uncommon along the rim of the Pacific Ocean, long known for its volcanic activity.
Geothermal steam or water has, in some regions of the world, been used for centuries for therapeutic treatment of physical infirmities and diseases. In other regions, such geothermal fluids have long been used to heat dwellings and in industrial processes. Although efforts to further develop geothermal resources for these site-restrictive uses continue, considerable recent research and development has been directed to the exploitation of geothermal resources for production of electrical power which can be conducted, often over existing power grids, for long distances from the geothermal sources. In particular, recent steep increases in the cost of petroleum products used for conventional production of electric power, as well as actual or threatened petroleum fuel shortages or embargos, have intensified the interest in use of geothermal fluids as an alternative, and generally self-renewing, source of power plant "fuel."
General processes by which geothermal fluids can be used to generate electric power have been known for some time. As an example, geothermal steam, after removal of particulate matter and such polluting gases as hydrogen sulfide and ammonia, can be used in the manner of boiler-generated steam to operate steam turbine generators.
Naturally pressurized geothermal brine or water having a temperature of over about 400.degree. F. can be flashed to a reduced pressure to convert some of the brine or water to steam. The steam produced in this manner can then be used to drive steam turbine generators. The flashed geothermal liquid and the steam condensate obtained from power generation are typically reinjected into the ground to replenish the aquifer and prevent ground subsidence. Cooler geothermal brine or water can often be used to advantage in binary systems in which a low-boiling point, secondary liquid is vaporized by the hot geothermal liquid, the vapor produced being used to operate gas turbine generators; again, the cooled brine is typically reinjected into the ground.
As might be expected, use of geothermal steam is preferred over use of geothermal water or brine for generating electric power because the steam can be used more directly, easily and cheaply. Consequently, where readily and abundantly available, geothermal steam has been used for a number of years to generate commercially important amounts of electric power at favorable costs. For example, even by the late 1970's, geothermal steam at the Geysers in Northern California was generating about two percent of all the electricity used in California.
While energy production facilities at important geothermal steam sources, such as at The Geysers, are generally still being expanded, the known number of important geothermal steam aquifers is small compared to that of geothermal brine or water. Current estimates are, in fact, that good geothermal brine or water sources are about five times more prevalent than are good sources of geothermal steam. The potential for generating electric power is, therefore, much greater for geothermal brine and water than it is for geothermal steam. As a result, considerable current geothermal research is understandably directed towards the development of economical geothermal brine and water electric power generating plants, much of this effort within the United States being expended towards use of vast geothermal brine resources in the Imperial Valley of southern California.
Although, as above-mentioned, general processes are known for using geothermal brine or water for production of electric power, serious problems, especially with the use of typically highly saline geothermal brine, are often encountered in practice. These problems are frequently so great as to prevent the production of electric power at competitive rates and, as a consequence, greatly impede the progress of flashed geothermal brine power plant development in many areas of the world.
These severe problems associated with the use of geothermal brines are principally caused by the usually complex composition of geothermal brines. At natural, aquifer temperatures in excess of about 400.degree. F. and pressures in the typical range of 400 to 500 psig, the brines leach large amounts of salts, minerals and elements from aquifer formations, the brines presumably being in chemical equilibrium with their formations. Thus, although brine composition may vary from aquifer to aquifer, wellhead brine typically contains very high levels of dissolved silica, as well as substantial levels of dissolved heavy metals such as lead, copper, zinc, iron and cadmium. In addition, many other impurities, particulate matter and dissolved gases are present in most geothermal brines.
As natural brine pressure and temperature are substantially reduced in power plant steam production (flashing) stages, chemical equilibrium of the brine is disturbed and saturation levels of impurities in the brine are typically exceeded. This causes impurities and silica to precipitate from the brine, as a tough scale, onto surrounding equipment walls and in reinjection wells, often at a rate of several inches in thickness per month. Assuming, as is common, that the brine is supersaturated with silica at the wellhead, in high temperature portions of the brine handling system, for example, in the high pressure brine flashing vessels, heavy metal sulfide and silicate scaling typically predominates. In lower temperatures portions of the system, for example, in atmospheric flashing vessels, amorphous silica and hydrated ferric oxide scaling has been found to predominate. Scale, so formed, typically comprises iron-rich silicates, and is usually very difficult, costly and time consuming to remove from equipment. Because of the fast growing scale rates, extensive facility down time for descaling operations, unless scale reducing processes are used, is often necessary. Associated injection wells may also require frequent and extensive rework due to scale buildup and new injection wells may periodically have to be drilled at great cost.
Therefore, considerable effort has been, and is being, directed towards developing effective processes for eliminating, or at least very substantially reducing, silica scaling in flashed geothermal brine handling systems. One such scale reduction process, disclosed in U.S. Pat. No. 4,439,535 to Featherstone, et al., involves the induced precipitation of scale-forming materials, principally silica, from the brine in the flashing stage by contacting the flashed brine with silica or silica-rich seed crystals. When the amount of silica which can remain dissolved in the brine is exceeded by the brine being flashed to a reduced pressure, silica leaving solution in the brine deposits onto the seed crystals. Not only do the vast number of micron-sized seed crystals introduced into the flashing stage provide a very much larger surface area than the exposed surfaces of the flashing vessels but also the silica from the brine tends to preferentially deposit onto the seed crystals for chemical reasons. Substantially all of the silica leaving the brine therefore, precipitates onto the seed crystals instead of precipitating as scale onto vessel and equipment walls and in injection wells.
Typically, the crystallized silica precipitate is settled from the brine in a downstream reactor-clarifier stage, the clarified brine overflow therefrom being flowed on to a filtering stage and then to a reinjection stage. Some of the silica precipitate (sludge) from the reactor clarifier may be pumped back upstream into the flash crystallization stage as seed material, the remainder being dewatered and removed from the facility for disposal. The amount of such silica sludge requiring disposal is, however, relatively large. For example, for a 10 megawatt power plant which requires a brine flow rate of about 1.3 million pounds an hour, more than six tons a day of silica sludge may be produced and require disposal.
During the silica crystallization process, many other materials precipitate from the brine onto the seed material along with the silica. The produced sludge, herein referred to as silica or silicious sludge, although mostly silica, also typically contains significant amounts of barite and heavy metals, such as lead, copper and zinc, which, above specific levels of concentration, may be considered as toxic and may therefore require disposal at specially designated toxic waste dumps. The costs associated with disposal of toxic silica sludge are substantial and can be expected to increase as additional and larger geothermal brine power plants are constructed and more silicious sludge is produced, as allowable concentrations of heavy metals in the sludge are reduced to meet anticipated stricter environmental requirements and as toxic waste dumps become fewer and/or more remotely located. Even when the silicious sludge produced is non-toxic, it may, nevertheless, contain small particles and/or polluting materials which may, in time, be eroded or leached out by rain or ground water, thereby creating environmental problems unless care is taken to properly dispose of the sludge. Such sludge disposal, even when the sludge is non-toxic, can be very costly.
It is, therefore, an object of the present invention to provide a process for using geothermal sludge to make a novel concrete material which can be used for construction purposes, the cementing material, used with the sludge to make the concrete, causing the small particles, impurities and contaminants in the sludge to be substantially fixed in the concrete.
Another object is to provide a novel composition of concrete using sludge from geothermal brine as a constituent thereof.
Additional objects, advantages, and features of the invention will become apparent to those skilled in the art from the following description.